Thermally activated warning system

ABSTRACT

An electrically actuated safety alarm system for detecting the presence of toxic gases for use alone or in combination with a conventional gas filter breathing apparatus. Spaced electrodes, at least one of which is coated with an electrically insulating material of low melting point (e.g., wax) project into an electrical conducting medium (e.g., activated charcoal) in a container and is connected in series with signaling means with an audio and/or visual signal. Chemical means in the container reacts with the toxic gases to generate heat and melt the wax to complete the electrical circuit between electrodes through the charcoal and activate signaling means to generate the signal.

United States Patent Wallace THERMALLY ACTIVATED WARNING SYSTEMInventor: Richard A. Wallace, 43 Kingscote Garden, Stanford, Calif.94305 Filed: Feb. 8, 1974 Appl. No.: 440,654

US. Cl. 340/237 R; 128/1426 Int. Cl. G08B 21/00; A62B 7/10 Field ofSearch... 340/237 R, 235, 279, 227 C;

128/1462, 1 R, 202, DIG. 17, 191, 193, 142, 142.4, 142.6; 337/416;55/274 Wallace 340/235 X Oct. 7, 1975 [S 7 ABSTRACT An electricallyactuated safety alarm system for detecting the presence of toxic gasesfor use alone or in combination with a conventional gas filter breathingapparatus. Spaced electrodes, at least one of which is coated with anelectrically insulating material of low melting point (e.g., wax)project into an electrical conducting medium (e.g., activated charcoal)in a container and is connected in series with signaling means with anaudio and/or visual signal. Chemical means in the container reacts withthe toxic gases to generate heat and melt the wax to complete theelectrical circuit between electrodes through the charcoal and activatesignaling rneans to generate the signal.

19 Claims, 5 Drawing Figures US. Patent Oct. 7,1975

wm w a Rm E a raw. M 5 m vi TI-IERMALLY ACTIVATED WARNING SYSTEMBACKGROUND OF THE INVENTION This invention relates to an electricallyactuated safety alarm system for detecting the presence of a threshholdlevel of predetermined toxic gases. This alarm system includes a signalof an audio or visual type. It can be used alone as an alarm system,say, in a chemical plant or mine or used in conjunction with aconventional gas filter breathing apparatus.

Gas filter breathing apparatus typically include canisters with layersof material of the following types, alone or in combination; (a)granular material for sorbing toxic gases, (b) catalyst for converting atoxic gas, such as carbon monoxide to a harmless one such as carbondioxide, or (c) a reagent for reacting with the toxic gas andneutralizing its toxicity. Such canisters generally are effective onlyat relatively low levels of toxic gas (e.g., 1 percent or less). Thus,low capacity chin-type canisters are recommended for use at toxic gaslevels below 0.5 percent. At higher levels of toxic gases, the materialin the canister either is dissipated in a relatively short period oftime or sorbs only part of the toxic gas. Thus, in an emergency,particularly where a lethal spill or leak of a hazardous gas occurs, thegas mask wearer does not realize that his cartridge canister has becomesaturated until he detects the odor during inhalation. By that time,such inhalation may cause permanent health damage or even death. Thewearer may not have sufficient time to leave the hazardous area andreturn to fresh air as breathing the toxic gas may causeunconsciousness.

Many of the above filter systems rapidly generate heat at highconcentration of toxic gases. One warning system presently employed isthat such heat causes the air inhaled by the wearer of the mask to beuncomfortably hot. However, this may be too late resulting in the aboveharmful effects. In addition, the wearer is likely to be so preoccupiedwith performance of his emergency function that he may not notice theheating of the canister until it is too late to leave the area. If thecanister is of the type that fits on the back of the gas mask wearer,the wearer is further handicapped in noticing increased temperature ofthe canister especially if he is under stress.

Another type of warning system for gas filter breathing apparatusutilizes a window indicator in the canister. Normally, in such a windowindicator, two pieces of paper of different color are located side byside in the window. One paper is treated chemically to change color asit absorbs moisture. When it has changed sufficiently in color to matchthe paper this should indicate that the chemical sorbent has lost orwill shortly loose its effectiveness. In order to make a properobservation of the colors, it is necessary that the window indicator beobserved in daylight. In addition, because of the position of the windowindicator, it is very difficult, if not impossible, for the wearer toobserve the indicator when the mask is being worn. Also, this system isnot able to instantaneously signal the gas mask wearer when the level oftoxicity exceeds the capacity of the gas mask. There is a need for asimple audio, visual or combination audio/visual warning system whichcan instantaneously signal the gas mask wearer of his sudden exposure tohigh levels of toxic gases.

There is believed to be no effective warning system in public use whichcan be positioned in various environments such as chemical plants whichare rapidly activated by the sudden release of toxic gases. There is aneed for such systems of an audio/visual type to warn not only thosepersons exposed to the toxic gases in the immediate vicinity of thealarm system but also those persons within hearing distance of the alarmsystem who would shortly be exposed to the gases if they do notimmediately leave the premises.

SUMMARY OF THE INVENTION AND OBJECT The electrically actuated safetyalarm system of the,

present invention is used for detecting the presence of a selectedthreshhold level of predetermined toxic,

gases to indicate a dangerously high concentration of such gases. Thealarm system includes a container and spaced apart electrodes with anelectrically conductive medium (e.g., activated charcoal) disposedbetween the electrodes. Chemical means for generating heat on contactwith the toxic gases in the container is disposed in gaseouscommunication with the surroundings and in thermal communication with atleast one of the electrodes. At least one of the electrodes is coatedwith an inert material, preferably wax, in the region of theelectrically conductive medium to provide a barrier against contactbetween that'electrode and the medium. The coating is characterized byhigh electrical resistance and a melting point less than the temperatureof heat generated in the canister by contact of a threshhold level ofthe toxic gas and the chemical means. Signaling means is connected inseries with the electrodes to activate an audio and/or visual signalwhen the coating is melted responsive to a dangerously highconcentration of toxic gases. This activation is caused by a substantialdrop in the resistance between electrodes. The electrically conductivemedium is preferably activated charcoal which thereby also serves as theheat generating chemical means when sorbing certain toxic gases. Also,such chemical means may be a coating on or impregnated into suchcharcoal granules either catalytic or directly reactive with the toxicgases or may be a layer having the same properties partitioned from thecharcoal. 7

The above alarm system may be in conjunction with chemical filterbreathing apparatus or independently as by mounting on a wall of achemical plant.

Generally, it is an object of the present invention to provide anelectrically actuated safety alarm system for detecting and signalingthe presence of a selected threshhold of predetermined toxic gases.

Another object of the invention is to provide an alarm system of theabove type which generates an audio or visual signal or a combination ofthe two.

It is a further object of the invention to provide an alarm system ofthe above type in combination with a chemical filter breathing apparatusto provide a warning to the wearer when the apparatus is insufficient tofilter predetermined levels of toxic gases.

It is another object of the invention to provide an apparatus of theabove type in which the electrical signal assembly is readily removedfor repeated use after exhaustion of the chemical systems.

It is another object of the invention to provide an apparatus of theabove type in which the alarm device is highly reliable, relativelyinexpensive, and can be easily manufactured.

It is a further object of the invention to provide an alarm device ofthe above character which can be adapted to chemical filter breathingapparatus already in the field.

It is a specific object of the invention to provide an alarm system withan-audio signal which can (a) clearly warn the wearer of a chemicalfilter breathing apparatus of the danger of high concentrations of toxicgases even where he is pre-occupied with emergency functions and (b)signal other persons if the wearer becomes unconscious as a result ofexposure to the gases.

It is another object of the invention to provide an alarm system whichcan be mounted for long-term usage in an area of potential danger forwarning persons in the vicinity of the sudden release of toxic gases.

Additional objects and features of the invention will be apparent fromthe following description in which the preferred embodiments are setforth in detail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a front view in perspectiveof a man wearing a chemical filter breathing apparatus with an alarmsystem according to the present invention.

FIG. 2 is an expanded front view partially broken away of a two probeelectrode alarm system of the present invention.

FIG. 3 is a circuit diagram of an audio/visual assembly suitable for thepresent invention.

FIG. 4 is a side view partially broken away of a single probe electrodealarm system in accordance with the present invention.

FIG. 5 is a schematic view of another embodiment of a single probeelectrode system.-

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The chemical filterbreathing apparatus portion of FIG. 1 is of a conventional chin typesuch as a type GMP produced by Mine Safety Appliances Company (MSA) ofPittsburgh, Pa. This device is recommended by MSA for respiratoryprotection against toxic gases and vapors in concentrations not inexcess of 0.5 percent by volume. This breathing apparatus is illustratedin conjunction with an alarm system in accordance with the presentinvention. It includes an electrically conductive canister 11 consistingof a drawn steel ovalshaped body 12 which has been copper-plated. Bottomand top closure wall 13 and 14 are provided for closing the open lowerand upper ends of body 12. The bottom closure wall 13 is provided with ascreen opening 16 which is sealable when the canister is not in use. Ina conventional breathing apparatus, a filter, not shown, is mounted inthe bottom of the canister for filtering particulate materials such astoxic dust and the like. A pipe 17 provides gas communication between anopening in the top wall 14 and facemask 18. The canister body 12 definesan open gas passageway through the canister so that the respiratorytract of the wearer is in communication with air from the environmentafter filtering through the canister.

Referring to FIG. 2, two different granular sorbent 19 materials areprovided within the canister 11 arranged in layers. The top layer 19 isformed of hopcalite, a mixture of copper and manganese oxides,conventionally used in gas masks as a catalyst to oxidize carbonmonoxide to carbon dioxide in the presence of oxygen in the air. Ascreen 20 is provided below the hopcalite and another screen, not shown,can be positioned thereabove to separate a suitable drying agent (e.g.,anhydrous calcium chloride) serving to prevent moisture from reachingthe hopcalite through inlet pipe 17. A sorbent layer 21 of activatedcharcoal is provided below screen 20 which is conventionally used tosorb organic vapors such as carbon tetrachloride. Such. activatedcharcoal also servesas an electrically conductive medium for purposes ofthe present invention.

Portions of the above conventional chemical filter breathing apparatusare utilized as an integral part of the safety alarm system of thepresent invention by means of the following additional apparatus.Electrode means is provided in the form of spaced apart electrodes 22and 23, suitably formed of copper, project through sorbent material 19.The electrodes are rigidly mounted to the canister wall with a portionof each one projecting outwardly therefrom. The probes are suitablymounted with a strong adhesive polymer, such as epoxy resin, withelectrical insulating properties. The adhesive layer 20 extends asufficient distance along the probe walls into the canister to prevent ashortcircuit between the electrodes through the canister wall.

ably of wax, is formed into a complete layer about electrodes 22 and 23particularly in the region of the electrically conductive medium,charcoal granules 21.

Coating 24 serves to provide a barrier against contact between saidelectrodes and charcoal granules. It is characterized by high electricalresistance and a melting point substantially above room temperature,say, at least 50C or more, for reasons to be described hereinafter. Thewax is characterized by chemical inertness, especially to water vapor,and is not hygroscopic. In general, the melting point of waxes increasewith molecular weight. A typical molecular weight for suitable waxes ison the order of 2,000.

Suitable waxes of the above type contain esters derived from long-chainalcohols and long-chain acids. For example, beeswax is largely myricylpalitate while carnauba wax includes myricyl cerotate. Other suitablewaxes include high molecular hydrocarbons, alcohols, and ketones. Themelting point of the wax should be selected to be above the maximumtemperature of the gaseous environment in which it is to be used (e.g.,above 45C in a temperate climate). On the other hand, the melting pointshould be below a temperature at which the exothermic reaction describedhereinafter occurs. Suitable waxes and their melting points are given inthe following table.

TABLE 1 Wax Melting Point (C) Referring again to FIG. 2, an inertcoating 24, suit- Carbowax 4000 50-57 (a polyethylene glycol ofmolecular weight 3000-3700) Ceramid C hlorowax l0l Paradox 93 Paraffin(hydrocarbon) 50-57 Ceresin (hydrocarbon) 65 MicrocrystallinePolyethylene Wax (Star S) Carnauba yellow 86 Camauba (Refined) 86 MontanI 70 Beeswax 62 Candelilla 73 The initial ohmic resistance of the waxcoating on the electrode is very high (e.g., greater than ohms). Aftermelting of the wax, the electrical circuit between the electrodes iscompleted through the carbon granules for a relatively low resistance(e.g., on the order of 1,000-100 ohms) for a net drop on the order of 10ohms. As set forth hereinafter, the circuit of the signaling means iscomplete at the lower ohmic resistance.

Waxes are particularly effective coatings for the electrodes inaccordance with the present invention because they are inexpensive, havehigh electrical resistance, melting points within the desired range, andare inert to most chemical reactants. It should be understood that otherorganic coating materials such as lowdensity polyethylene may beemployed so long as they have the above desired characteristics.

An electrically conductive medium is disposed in the canister betweenthe two electrodes. The medium is preferably a form of carbon as it iseffective and inexpensive. For example, activated charcoal is alreadypresent as a sorbent layer in a conventional canister for a chemicalfilter breathing apparatus as set forth above. Thus, the two electrodesneed only contact this preexisting layer.

Chemical means is provided in the canister for generating substantialheat when contacted with selected threshhold levels of the predeterminedtoxic gases in an exothermic chemical reaction. Such chemical means isin gaseous communication with the outside of the canister and in thermalcommunication with at least one coated electrode. In the embodiment ofFIG. 2, such chemical means comprises sorbent 19 in a form of hop calitewhich acts a catalyst for the highly exothermic oxidation of carbonmonoxide to carbon dioxide. As set forth above, the chemical meanscomprising sorbent 19 is disposed in a layer across the canister gaspassageway for contact by air passing between the surroundings andfacemask. When there are sufficiently high toxic gas levels in thesurrounding environment (e.g., at least 0.4 percent by volume of carbonmonoxide), sorbent 19 generates enough heat to the thermally conductivecanister and contents to melt the wax coating in the compartmentcontaining layer 21 of activated charcoal. It has been found that anexcellent electrically conductive bridge is formed between the twoelectrodes through the carbon granules when the wax has melted exposingthe electrodes. Conductivity is materially assisted by the phenomenonthat the charcoal granules adjacent the electrodes adhere to the wax asit melts to expose the electrodes.

Referring again to the drawing, signaling means generally denoted as 25includes electrical circuitry contained in housing 26. The circuitry iselectrically insulated within the housing. The signaling means 25includes an internal socket or other suitable clamping mechanism forreceiving the electrodes into the circuit. Signaling means 25 can bedetached from the electrodes when desired as after the chemical reagentsin the canister are expended by use. Thus, the removable signaling meanscan be used repeatedly with different canisters. Signaling means 25 maygenerate a signal of either audible sound or visible light such as lamp27, or both. The drop in resistance to trigger the audio/visual alarm issatisfied by melting of the wax in the aforementioned manner so that anelectrically conductive bridge is formed between electrodes 22 and 23 bydirect contact with charcoal granules 21.

Suitable signaling means 25 to generate an audio and/or visual alami inresponse to a dangerously high concentration of toxic gases isillustrated in FIG. 3. Lines 28 and 29 are coupled to electrode 22 and23 as discussed previously. When the electrical insulating barrier ofone electrode is still intact electrical resistance produced betweenlines 28 and 29 is relatively high; for example, greater than 10 ohms.Thus, the current flow caused by the positive potential of the battreyBl through the series circuit of R4, R5 and the high effectiveresistance between lines 28 and 29 is very low. The base input oftransistor O3 is very close to the plus battery potential and ismaintained in an off condition. The collector of Q3 is connected tolight emitting diode D1 and is also coupled to an oscillator circuitwhich includes transistors Q1 and Q2, the resistors R1, R2, R3, and thecapacitor C1.

In operation when the insulating coating on one of the electrodes ismelted due to the generation of heat caused by a dangerously highconcentration of toxic gases the resistance between lines 28 and 29 isreduced. When this resistance reaches a certain point, for example 5000ohms, transistor switch O3 is turned on which applies power to thevisual alarm light emitting diode D1 and at the same time activates theoscillator through resistor R3 to the base of transistor 01. Theoscillator circuit oscillates at an audio frequency which is convertedto an audio signal by means of a suitable electro-mechanical transduceror loud-speaker which is tapped off between 02 and line 29. Inaccordance with the invention even if the user of the device does notsee the visual signal he is notified by the audible sound.

In the embodiment illustrated in FIG. 2, coatings are provided upon bothelectrodes. It should be understood that it is only necessary to coatone electrode as that will create the high resistance which preventscompletion of the electrical circuit between the electrodes. Also, thewax coating only need be present in the area of the charcoal or otherelectrically conductive medium. Thus, where there are two compartments,as in FIG. 2, only the portion of the electrodes below screen 20 need becoated.

Referring to FIG. 4, another embodiment of the alarm system of thepresent invention is illustrated with the electrode means in differentform. A canister 40 of the same general type as described with respectto FIG. 2 is provided. A single layer of sorbent material 41 is providedrather than the two layers illustrated in FIG. 2. Electrode 42 extendsinto sorbent material 41. The other electrode comprises the housing 43of canister 40 which is formed of a conductive metal. Signaling meanshousing 44 is fitted with an electrically conductive finger 45 inelectrical communication with canister wall 43. Finger 45 is connectedto the same portion of the circuit of the signaling means as one of theelectrode probes 22 or 23 in FIG. 2. An inert coating 46 of the type setforth above is deposited on the surface of electrode 42.

In the embodiment of FIG. 4, sorbent 41 serves two purposes. It is thechemical means which generates sufficient heat on contact withthreshhold levels of the predetermined toxic gases to melt coating 46.In addition, it is the electrically conductive medium between electrodeswhich enables the alarm system to be activated upon melting of thecoating. Activated charcoal is capable of performing both of thesefunctions. This is to be contrasted with the embodiment of FIG. 2wherein the chemical means for generating heat and the electricallyconductive medium are in two separate compartments.

It is apparent that in the present system the chemical means mustgenerate sufficient heat on contact with the threshhold level of toxicgases to melt the coating in order to activate the alarm system. Contactof certain gases with activated charcoal alone is sufficient to generatethe requisite heat. Such gases include hydrogen sulfide and chlorinatedsolvents such as carbon tetrachloride or perchloroethylene.

Thus, the activated charcoal is capable of serving both as the chemicalmeans to generate heat and the electrically conductive layer. The sizesof the granular activated charcoal or other layers of particulatematerial used in accordance with the invention are the same asconventionally used in the gas mask industry. The size is greater thanthat which would present any undue pressure drop to gas flow. A suitablesize is 8-12 mesh.

In an alarm system suitable for toxic gases which do not yieldsufficient heat upon sorption by activated charcoal alone, suitablechemical means may be impregnated into the charcoal which generates therequisite amount of heat to melt the coating upon contact with theselected threshhold level of toxic gases. For example,'activatedcharcoal impregnated with silver or copper chromate has been found togenerate sufficient heat when contacted with alkyl (e.g., methyl, ethylor butyl) mercaptans and disulfides, cyanogen, phosgene, hydrocyanicacid and chloropicrin. Also, impregnating activated charcoal withpotassium iodide significantly increases the heat generated by contactwith hydrogen sulfide. Other chemical means which generate sufficientheat on contact with selected toxic gases may also be impregnated intothe charcoal.

Referring to FIG. 2, a system has been described in which the chemicalmeans for generating heat, hopcalite, is independent from theelectrically conductive medium. It should be understood that a heatgenerating substance other than hopcalite may be used in such systernsso long as the substance generate the requisite amount of heat whencontacted with the threshhold level of predetermined toxic gases. Forexample, the chemical means may comprise granules of a solid base suchas sodium hydroxide or calcium hydroxide or a combination of the formerwith calcium oxide (known as soda lime), such materials areexothermically reactive with threshhold levels of acidic gases togenerate sufficient heat to melt thewax coating. Such gases aregenerally strongly acidic and include hydrogen sulfide, hydrogenchloride, and sulfuric acid. Sulfur dioxide in a humid or moistatmosphere which converts to sulfurous acid will also trigger the alarmsystem.

As set forth above, the canisters may serve as the base for mounting thesignaling means of the present invention by drilling a hole in thecanister wall and mounting at least one electrode to project into theactivated charcoal compartment. However, other electrically conductivemedia may be used in place of activated charcoal, such as untreatedcarbon granules or metal particles. These would suffer from thedisadvantage that they would not perform the sorbent function describedabove. As set forth above, chemical means for generating heat other thanactivated charcoal also could be employed as the electrically conductivemedium, if upon melting of the wax, a sufficiently low re- Sistance ispresented to the flow of electricity to activate the alarm system.

The foregoing alarm system is illustrated with the electrode projectingthrough the top of the canister. It should be understood that suchelectrodes could also project through other portions of the canisterhousing where a single layer of activated charcoal carries the chemicalmeans for generating heat and serves as the electrically conductivemedium. With separate layers, the electrodes must project into theelectrically conductive medium and be in thermal communication with thechemical means for generating heat. Referring to FIG. 2, the electrodescould project through the side wall of the canister adjacent to theactivated charcoal. The thermal communication between the heat generatedby the upper sorbent layer is provided by both layers of granularmaterials as well as the metallic wall of the canister. I

The foregoing description relates to a chemical filter breathingapparatus of the chin canister type. However, it should be understoodthat the invention is applicable to larger canisters of both front andback mounted type. An audio alarm system is particularly beneficial forthe chin-type or back-mounted canister because of the difficulty for thewearer of the gas mask to detect the visual warning signal.

When the alarm system is used in conjunction with a chemical filterbreathing apparatus, it serves to warn the wearer of dangerously highlevels of toxic gases beyond the capacity of the breathing apparatus.The exact threshhold level of toxic gases .which will trigger thepresent alarmsystem is dependent upon a number of factors including thelevel of exothermic heat generated by the chemical means. However, itcan be generalized that in many systems including those of the typedescribed herein, such threshhold level is on the order of l4 percenttoxic gas. Even this minimum level is beyond the capacity of theforegoing type of breathing apparatus, especially of the small chin-typecanister.

The foregoing electrically actuated safety alarm systern is described interms of signaling means attached to a canister of a chemical filterbreathing apparatus. The invention in its broadest aspect includes theuse of the alarm system independently of the chemical filter breathingapparatus. In this case, of course, no facemask or passageway to thesame is required. Instead, a simple canister of the general type used inthe breathing apparatus is either independently constructed or thefacemask portion is removed from the breathing apparatus. Otherwise, thealarm system is essentially the same as the one described above.

An independent alarm system of the foregoing type may be installed inany environment of potential toxic gas presence where people mightgather or be near. It can be used to warn of a sudden massive leak orproduction of toxic gases to warn people to leave the area. For example,it could be employed in a chemical plant, coal mine, or the like. Anaudio alarm system is particularly effective to provide a warning inthis type of environment.

It should be understood that it may require either a longer time or ahigher concentration of toxic gases to activate the independent alarmsystem than would be required to activate the alarm system utilized inconjunction with the chemical filter breathing apparatus. This isbecause in the latter case the air and toxic gases are rapidly drawnpast the chemical means for generating heat during lung inflation. Thisis to be contrasted with the stationary independent canister, say,mounted upon a wall of a chemical plant, which relies upon permeation oftoxic gases in relatively stagnant air into the canister which contactthe chemical means. It is apparent that a given level of toxic gases inthe atmosphere will generate heat at a rate dependent upon the rate ofcontact with the chemical means.

To decrease the response time of the above independent alarm system in astagnant atmosphere, an aspirator bulb or a pump can be used to draw thesurrounding air (which may contain toxic gas) into the canister. For

example, a small four-cylinder electric pump is capable of drawing a 1percent carbon monoxide in air mixture into the canister at a rate of 4liters per minute.

Referring to FIG. 5, another embodiment of the invention of the generaltype set forth in FIG. 4, is illustrated in compact form particularlysuitable for use in limited space such as a flue or gas conduit or thelike. Instead of a canister, the device includes a perforate cylindricalcontainer 50 formed of an electrically conductive material such as acopper mesh screen. Electrode 51 extends into activated charcoal layer52 and is rigidly mounted to container 50 with a layer 53 of a suitableelectrically insulating adhesive such as epoxy resin as set forth above.Container 50 forms the other electrode. Signaling means is provided ofthe type set forth above and includes a housing 54. Electrical leads 56and 57 connect and are attached to electrode 51 and container 54,respectively, to provide communication with the appropriate portion ofthe circuit in housing 54. A coating 58 of the type set forth above isdeposited on the surface of electrode 51.

The embodiment of FIG. 5 is well adapted to placement in a conduit offlowing gas. The signaling means is remote from the conduit as in acentral control panel. Also, the open mesh of container 50 exposes thechemical component of the system to the flowing gas more rapidly than inthe generally solid canister. Furthermore, the container sizing can besmall enough, (e.g., l inch diameter by 6 inch length), to fit in aconfined space.

An independent alarm system of the type set forth in FIG. 5 is welladapted for mounting in a flue or conduit at the exit from an internalcombustion engine, an oil refinery, a coal combustion operation, blastor open hearth furnace and the like for actuation of the alarm systemupon sensing of toxic gases in excess of the predetermined level. It isparticularly useful in the above environments for monitoring thecompleteness of combustion of the carbonaceous fuel.

The alarm system of the present invention is particularly economicalbecause it can be utilized in conjunction with a canister for aconventional gas mask as of the type manufactured by MSA. For example,in a dual electrode system, the electrodes are mounted to the canisterwall to contact the signaling means. Alternatively, in a singleelectrode system, only one electrode probe is mounted in the canisterwall since the other electrode is provided by the canister wall itself.Whether the alarm system is used alone or in conjunction with thechemical filter breathing apparatus, the signaling means is readilymounted to a conventional canister used in such apparatus. Similarly,after the alarm system is triggered, the signaling means can be removedfrom the used canister and is reusable with fresh canisters. Also, theembodiment of the invention in which the container is a wire mesh basketor the like is inexpensive to construct. I i

In order to more clearly disclose the nature of the present invention,specific examples of its practice are herein given. It should beunderstood, however, that this is done by way of example and is notintended to limit the scope of the appended claims.

EXAMPLE I A chin-type canister having separate compartments for chemicalmeans and activated charcoal was utilized. Specifically, the gas maskwas of a GMC type manufactured by MSA with a chemical means in an uppercompartment of soda lime mixture and a lower compartment of activatedcharcoal. A single copper probe coated with paraffin wax (melting point53-54C) was inserted projecting through the soda lime and into thecharcoal granules. The distance between the copper probe electrode andthe side of the canister serving as the other electrode was on the orderof 0.5 inches. The signaling means had an audio/visual signal of thetype described herein. The toxic gas in the atmosphere was poisonousphosgene at a level of 0.2 percent by volume. The breathing rate of thewearer was about 20 liters of air per minute at 25C. The wax melted andthe audio/visual alarm was activated by the time the canister reached atemperature of about 57C.

EXAMPLE 2 A chin-type gas mask canister of the GMP type manufactured byMSA was employed with the general construction of FIG. 1. The onlydifference was that the charcoal layer was impregnated with copperchromate. A breathing rate of 15 liters per minute and a toxic gas levelof 200 ppm was utilized in two different runs. In one run the toxic gaswas hydrocyanic acid and in the other run was chloropicrin. Both gasesgenerated highly exothermic reactions in a short time and activated thealarm.

The above experiments were repeated using a chin type gas mask canisterof the GMR type also manufactured by MSA with the same results.

EXAMPLE 3 In this case, a type GMC-SS-l gas mask canister manufacturedby MSA was utilized. Middle and bottom layers of potassiumiodide-impregnated charcoal were used with a top layer of soda lime. Theapparatus was the same as that of Example 1.

A breathing rate of about 30 liters per minute was used for airincluding 1.5 percent of hydrogen sulfide at 25C. After approximately 4minutes, the canister heated to a temperature of about 68C. By thistime, the paraffin wax coating on the electrode melted and theelectrical circuit was completed. The ohmic resistance dropped sharplyfrom about 10*" ohms to about 10 ohms. It was noted that the melted waxprovided an adhesive for the bonding of a large number of carbongranules to the inserted metal probe. This phenomenon is believed tolower the resistance after wax is melted.

EXAMPLE 4 A canister of the type generally set forth in Example I usedwhere there is a danger of carbon monoxide is used. The canister is typeN manufactured by MSA which includes hopcalite as an upper layer servingas a catalyst to convert carbon monoxide to carbon dioxide in thepresence of air. The highly exothermic reaction with carbon monoxiderapidly raised the temperature of the canister to as high as 94C in thepresence of a gas mixture at the above breathing rate including 0.4

percent by volume carbon monoxide. Prior to reaching this temperature,the paraffin wax coating on the electrode has melted activating theaudio/visual alarm.

EXAMPLE A canister with a single layer of activated charcoal illustratedin FIG. 1 was employed. The particular canister was a type GMA producedby MSA filled with activated charcoal granules (8-l2 mesh). Carbontetrachloride in air at a level of 2 percent by volume were inhaled at abreathing rate of about 25 liters per minute. After 8 minutes, thecanister attained a temperature of about 75C and the coating of paraffinwax melted to set off the alarm. The ohmic resistance dropped sharplyfrom ohms to approximately 10 ohms.

The above experiment was repeated except that the carbon tetrachloridewas in an air stream of 85 percent relative humidity. The onlydifference in results was that the alarm was actuated at a slightlylonger time, about ll minutes.

EXAMPLE 6 Breathing tests were performed with a type N gas mask canistermanufactured by MSA. In this case a dual copper electrode probe wasutilized with the signaling means, both of which were coated withpolyethylene wax. The electrodes were spaced about 0.5 inches apart.They projected through the hopcalite layer into the activated charcoallayer.

The warning system was utilized for the detection of about 0.6 percentby volume of carbon monoxide in an air mixture at a breathing rate ofabout 30 liters at room temperature. After about minutes, thetemperature of the canister reached about 90C at which time the wax hadmelted. The ohmic resistance de-' creased from about 10'" ohms to about10 ohms. This reduction was sufficient to activate the warning system.

EXAMPLE 7 The preceding experiment was repeated with the exception thatthe warning system was of a single electrode with the canister servingas the ground electrode. After about 20 minutes of inhalation, the metalcanister reached the same temperature and the wax coating melted. Thefinal ohmic resistance after melting was about 500 ohms.

EXAMPLE 8 A single inserted wax coated copper probe was utilized with achin type GMP gas mask cartridge manufactured by MSA. A single layer ofcharcoal granules impregnated with copper chromate salts is employed.Gases such as methyl, ethyl or butyl or mercaptans and disulfidesundergo exothermic oxidation-reduction reactions in the presence of thesalts as they are sorbed by the activated charcoal. The temperature ofthis reaction is sufficient to melt the wax coating and therebyactivates the warning system at a concentration of 0.3 percent by volumein air of the foregoing toxic gases.

EXAMPLE 9 An alarm system was employed including a single copper probecoated with wax (melting point C) inserted into a lower layer ofgranular activated charcoal and an upper layer of hopcalite catalystboth layers being surrounded by a cylindrical copper ground screen. Anaudio alarm of the type described herein was employed. This alarm wasused to detect a hazardous concentration of 1 percent by volume ofcarbon monoxide in stagnant air of 50 percent relative humidity. Onexposure of the hopcalite to this carbon monoxide containing atmospherefor 14 minutes the canister reaches a temperature of 68C and the waxmelted. Thereafter, the activated charcoal granules (8-12 mesh size)made good electrical contact with the bare copper probe to activate theaudio alarm.

To decrease this response time for the alarm, a small electric pump wasattached to the canister. This pump forced the passage of the same gasthrough the hopcalite layer wherein a rapid exothermic reactionoccurred. The response time to activate the audio signal was reduced toabout 6 minutes.

EXAMPLE 10 An alarm system of the type set forth in Example 3 was placedwithin a four-inch internal diameter pipe or conduit through which a 0.3percent by volume hydrogen sulfide in dry air was flowing at a rate of20 liters per minute at 20C. The paraffin wax used had a melting pointof 65C.

In order to simulate an emergency condition, the concentration ofhydrogen sulfide in this flowing gas mixture was suddenly increased to alevel of about 2 percent by volume. The reaction of this high level ofhydrogen sulfide with the catalyst in the canister rapidly increased thecanister temperature to 71C in about 4 minutes. This melted the wax andactivated the audio/visual alarm.

I claim:

1. In an electrically actuated safety alarm system for detecting andsignaling the presence of a selected threshhold level of predeterminedtoxic gases, a container; electrode means in the form of spaced apartelectrodes; an electrically conductive medium disposed in said containerbetween said spaced apart electrodes; chemical means for generatingsubstantial heat on contact with threshhold levels of the saidpredetermined toxic gases and disposed in said container in gaseouscommunication with the outside of the container and in thermalcommunication with at least one of said electrodes; an inert coating onat least said last named one electrode disposed in the region of saidelectrically conductive medium serving to provide an electricallyinsulating barrier against contact between said one electrode and saidmedium, said coating being characterized by high electrical resistanceand a melting point no greater than the temperature of heat generated insaid container by contact of said chemical means with a threshhold levelof a predetermined toxic gas; signaling means connected in series withthe electrodes and including means for generating a signal responsive tothe melting of said coating whereby said resistance is substantiallyreduced.

2. An alann system as in claim 1 in which said coating 18 a wax.

3. An alarm system as in claim 1 in which said signaling means includesaudible sound generating means.

4. An alarm system as in claim 1 in which said signaling means includesa lamp.

5. An alarm system as in claim 1 in which said electrically conductivemedium comprises a layer of granular material in the container.

6. An alarm system as in claim 5 in which said medium comprises carbongranules.

7. An alarm system as in claim 5 in which said electrically conductivelayer and chemical means comprise activated charcoal.

8. An alarm system as in claim 5 in which said chemical means isdeposited onto the granules of said electrical conductive layer.

9. An alarm system as in claim 5 together with a partition and whereinsaid chemical means comprises a layer of heat generating materialseparated by said partition from said electrically conductive layer.

10. An alarm system as in claim 5 in which said electrode means is inthe form of one electrode extending into the electrically conductivelayer and the other electrode in the form of a body with theelectrically conductive layer disposed in the body.

11. An alarm system as in claim 5 in which said electrodes comprise twospaced apart probes extending into the electrically conductive layer.

12. An alarm system as in claim 1 in which said signaling means isdetachable from said container.

13. In a chemical filter breathing apparatus with electrically actuatedsafety alarm system for detecting and signaling the presence of aselected threshhold level of predetermined toxic gases, a facemask; acontainer with a gas passageway between the surroundings and thefacemask; electrode means in the form of spaced apart electrodes;electrically conductive medium disposed in said container between saidspaced apart electrodes, chemical means for sorbing the predeterminedtoxic gases and for generating heat on contact with threshhold levels ofsaid predetermined toxic gases,

said chemical means being disposed in said container passageway and inthermal communication with at least one of said electrodes; an inertcoating on at least said last named one electrode disposed in the regionof said electrically conductive medium serving to provide a barrieragainst contact between said one electrode and the electricallyconductive medium, said coating being characterized by high electricalresistance and a melting point less than the temperature of heatgenerated in said container by contact of said chemical means with athreshhold level of the predetermined toxic gas in the containerpassageway; signaling means connected in series with the electrodes andincluding means for generating a signal responsive to the melting ofsaid coating whereby the resistance between electrodes is substantiallyreduced.

14. A chemical filter breathing apparatus as in claim 13 in which saidcoating is a wax.

15. A chemical filter breathing apparatus as in claim 13 in which saidsignaling means includes audible sound generating means.

16. A chemical filter breathing apparatus as in claim 13 in which saidelectrically conductive medium comprises a layer of carbon granularmaterial in the canister.

17. A chemical filter breathing apparatus as in claim 16 in which saidcarbon granules and chemical means comprise activated charcoal.

18. A chemical filter breathing apparatus as in claim 16 in which saidchemical means is deposited onto the carbon granules.

19. A chemical filter breathing apparatus as in claim 16 together with apartition and wherein said chemical means comprises a layer of heatgenerating material separated by said partition from said carbongranules.

1. In an electrically actuated safety alarm system for detecting andsignaling the presence of a selected threshhold level of predeterminedtoxic gases, a container; electrode means in the form of spaced apartelectrodes; an electrically conductive medium disposed in said containerbetween said spaced apart electrodes; chemical means for generatingsubstantial heat on contact with threshhold levels of the saidpredetermined toxic gases and disposed in said container in gaseouscommunication with the outside of the container and in thermalcommunication with at least one of said electrodes; an inert coating onat least said last named one electrode disposed in the region of saidelectrically conductive medium serving to provide an electricallyinsulating barrier against contact between said one electrode and saidmedium, said coating being characterized by high electrical resistanceand a melting point no greater than the temperature of heat generated insaid container by contact of said chemical means with a threshhold levelof a predetermined toxic gas; signaling means connected in series withthe electrodes and including means for generating a signal responsive tothe melting of said coating whereby said resistance is substantiallyreduced.
 2. An alarm system as in claim 1 in which said coating is awax.
 3. An alarm system as in claim 1 in which said signaling meansincludes audible sound generating means.
 4. An alarm system as in claim1 in which said signaling means includes a lamp.
 5. An alarm system asin claim 1 in which said electrically conductive medium comprises alayer of granular material in the container.
 6. An alarm system as inclaim 5 in which said medium comprises carbon granules.
 7. An alarmsystem as in claim 5 in which said electrically conductive layer andchemical means comprise activated charcoal.
 8. An alarm system as inclaim 5 in which said chemical means is deposited onto the granules ofsaid electrical conductive layer.
 9. An alarm system as in claim 5together with a partition and wherein said chemical means comprises alayer of heat generating material separated by said partition from saidelectrically conductive layer.
 10. An alarm system as in claim 5 inwhich said electrode means is in the form of one electrode extendinginto the electrically conductive layer and the other electrode in theform of a body with the electrically conductive layer disposed in thebody.
 11. An alarm system as in claim 5 in which said electrodescomprise two spaced apart probes extending into the electricallyconductive layer.
 12. An alarm system as in claim 1 in which saidsignaling means is detachable from said container.
 13. In a chemicalfilter breathing apparatus with electrically actuated safety alarmsystem for detecting and signaling the presence of a selected threshholdlevel of predetermined toxic gases, a facemask; a container with a gaspassageway between the surroundings and the facemask; electrode means inthe form of spaced apart electrodes; electrically conductive mediumdisposed in said container between said spaced apart electrodes,chemical means for sorbing the predetermined toxic gases and forgenerating heat on contact with threshhold levels of said predeterminedtoxic gases, said chemical means being disposed in said containerpassageway and in thermal communication with at least one of saidelectrodes; an inert coating on at least said last named one electrodedisposed in the region of said electrically conductive medium serving toprovide a barrier against contact between said one electrode and theelectrically conductive medium, said coating being characterized by highelectrical resistance and a melting point less than the temperature ofheat generated in said container by contact of said chemical means witha threshhold level of the predetermined toxic gas in the containerpassageway; signaling means connected in series with the electrodes andincluding means for generating a signal responsive to the melting ofsaid coating whereby the resistance between electrodes is substantiallyreduced.
 14. A chemical filter breathing apparatus as in claim 13 inwhich said coating is a wax.
 15. A chemical filter breathing apparatusas in claim 13 in which said signaling means includes audible soundgenerating means.
 16. A chemical filter breathing apparatus as in claim13 in which said electrically conductive medium comprises a layer ofcarbon granular material in the canister.
 17. A chemical filterbreathing apparatus as in claim 16 in which said carbon granules andchemical means comprise activated charcoal.
 18. A chemical filterbreathing apparatus as in claim 16 in which said chemical means isdeposited onto the carbon granules.
 19. A chemical filter breathingapparatus as in claim 16 together with a partition and wherein saidchemical means comprises a layer of heat generating material separatedby said partition from said carbon granules.